1. Field of the Invention
The present invention relates to a power semiconductor device, and more particularly, to a power semiconductor device of high breakdown voltage that has a termination structure.
2. Description of the Background Art
In order to relax an electric field at an end of a depletion layer in the vicinity of a main junction surface in a termination of a power semiconductor device of high breakdown voltage such as diodes and bipolar transistors having high breakdown voltage, power metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs), there is a technology of providing a field plate structure in which a conductive film (field plate) is provided on a semiconductor substrate through an insulating film and providing a reduced surface field (RESURF) layer obtained by selectively providing a low-concentration diffusion layer of conductivity type different from that of the substrate in the vicinity of the surface of the semiconductor device, to thereby improve device breakdown voltage. In the field plate structure, for example, an Al—Si film is provided on the RESURF layer through an oxide film. When the field plate structure is provided on a junction region of the RESURF layer and a p-type layer in the vicinity of the main junction region on which an electric field is likely to be particularly concentrated, an electric field in the vicinity of the main junction region can be relaxed to improve device breakdown voltage.
In a case where an insulating film under the field plate has a small thickness in the field plate structure, avalanche occurs at an end of the field plate and a device breakdown voltage drops, and thus the insulating film is required to have a large thickness. However, formation of the insulating film causes a step in a wafer process (an entire film thickness becomes larger compared with the region in which the insulating film is not formed). Accordingly, a larger film thickness of the insulating film causes unevenness in application of a resist when the resist is applied, leading to a problem that, for example, a focus margin becomes difficult to be ensured in photolithography.
In order to solve the above-mentioned problem, the applicant(s) of the present application have proposed the termination structure in which an insulating film under a field plate is formed in a semiconductor substrate. The termination structure is formed by forming an insulating film on an entire surface of a semiconductor substrate after the formation of a recessed region in the semiconductor substrate, and planarizing the surface after the film formation by chemical mechanical polishing (CMP). Further, there is a termination structure in which a recessed region is formed in a RESURF layer formed on a surface of a semiconductor substrate and an insulating film is filled in the recessed region (for example, see Japanese Patent Application Laid-Open No. 2009-88385).
In the termination structure in which the recessed region is provided and an insulating film is formed in the recessed region, in order to prevent damage on the semiconductor substrate, it is required to leave an insulating film formed on a region (non-recessed region) other than the recessed region including a transistor cell region in performing CMP process after the formation of the insulating film on an entire surface of the semiconductor substrate. However, the distribution of thickness of the insulating film after the CMP process depends on an area of the non-recessed region surrounded by the recessed region, and the distribution of film thickness increases as the area of the non-recessed region increases (unevenness in film thickness increases). Large unevenness in film thickness leads to unevenness in characteristics.